PCT Application No. W/O 95/10046, describes optical measuring arrangements and measuring methods for measuring a periodic quantity, in particular for measuring a magnetic alternating field or an electric a.c. current, utilizing the magnetooptic Faraday effect, or for measuring an electric alternating field or an electric a.c. voltage utilizing the electrooptical Pockels effect. Polarized measuring light is coupled into a sensor device that is under the influence of the periodic quantity. The polarization of the measuring light is varied in the sensor device as a function of the periodic quantity. To analyze this change in polarization, after propagating at least once through the sensor device, the measuring light is split into two linearly polarized partial light signals having different polarization planes. An intensity-normalized signal P is formed, which corresponds to the quotient of a difference and the sum of the light intensities of the two partial light signals. A temperature-compensated measuring signal is derived from an alternating signal component and from a direct signal component of the intensity-normalized signal. In this context, the direct signal component does not contain any frequency components of the periodic quantity and is only used for temperature compensation.
"Optical Combined Current & Voltage H.V. Sensors, GEC Alsthom, T&D, describes a magnetooptical current transformer in which a light signal that is linearly polarized in a polarizer propagates through a Faraday glass ring and is then split by a polarizing beam splitter into two partial light signals, which are linearly polarized, transversely with respect to one another (two-channel polarization analysis). Each of the two partial light signals is fed via an optical fiber to a corresponding photodiode, which converts the partial light signal in question into an electric intensity signal S1 or S2, which is proportional to the light intensity of the corresponding partial light signal. Due to the different attenuation in the two optical fibers, the two proportionality constants can differ from one another at this point. To compensate for these differences in responsivity, provision is made for a special closed-loop control. A controllable first amplifier connected downstream from the first photodiode amplifies the intensity signal S1 by a corresponding gain K1, and a second amplifier connected downstream from the second photodiode amplifies the second intensity signal S2 by a second gain K2. At this point, direct signal components (DC values) of the two intensity signals S1 and S2 are determined, and the difference between the two direct signal components is set to zero by controlling the gain K1 of the first amplifier. From the two intensity signals K1.multidot.S1 and K2.multidot.S2, which are generally amplified with varying intensity, at the outputs of the two amplifiers, a measuring signal is now formed, which corresponds to the quotient (K1.multidot.S1-K2.multidot.S2)/(K1.multidot.S1+K2.multidot.S2) of the difference and the sum of the output signals of the amplifiers.